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A North Slope security guard shot and killed a polar bear earlier this month after the bear resisted efforts to drive it away from employee housing at BP's Endicott oil field.

The death appears to have been accidental, according to BP Alaska spokesman Steve Rinehart, who said the guard thought he'd fired a bean bag round at the female bear but BP later discovered it was a "cracker shell" that mortally wounded her.

The polar bear death is the first time in 35 years of working on the North Slope that a bear has been killed by a security guard working for BP, Rinehart said.

"We dearly wish it had not happened," Rinehart said, "but it's not a trend or a population impact. We have worked safely and carefully around polar bears under strict guidance from the U.S. Fish and Wildlife Service."

The Fish and Wildlife Service is investigating the incident, but spokesman Bruce Woods said he couldn't talk about the case because it is under investigation.

"We are taking it very seriously," Woods said.

Polar bears are listed as threatened under the Endangered Species Act and more than 187,000 square miles of the Arctic coast, including the North Slope, has been designated as critical habitat for the bears. Oil companies operating in the bears' environment must meet certain requirements aimed at disrupting the bears as little as possible in order to get federal permits.

Here's the story according to Rinehart:

Late in the evening of Aug. 3, a security guard, employed by Purcell Security, saw what turned out to be a female polar bear walking down the Endicott causeway and headed for an employee housing area. The guard flashed his vehicle lights at the bear, honked his horn and sounded his siren but the bear would not leave the area and instead approached the vehicle and began to act aggressively.

The guard pulled out his 12-guage shotgun and fired what he thought was a bean bag round at the bear. The less-lethal ammunition is designed to hit the bear in the hind quarters and drive it away.

The bear did run off at that point and BP reported the incident to the Fish and Wildlife Service, as required.

But a few days later, the bear returned, swimming off to the west and ending up on a shallow island area near the four-mile long causeway and 30-acre gravel drilling pad.

BP workers could see the bear through binoculars and continued to monitor it. But sometime between the night of Sunday, Aug. 14 and Monday morning, Aug. 15, they realized the bear was dead.

Fish and wildlife officers and BP security took a boat out to the island and examined the bear. That's when they discovered she had been shot with a cracker shell that had penetrated her side. A field necropsy indicated she died from internal injuries.

The bear carcass was towed into deep water and disposed of so as not to attract other bears.

Rinehart said he didn’t know why the security guard didn't realize the mistake at the time.

The guard has since been reassigned to a post off the Slope, Rinehart said.

Purcell Security officials did not return a call seeking comment for this story.

The cracker shell is designed to be fired in front of a bear. It explodes and makes a noise that is intended to startle the bear and scare it away, he said. It's not supposed to hit the bear.

The measures used to drive bears away from people and work sites are called "hazing" and guards are trained in hazing protocols. Rinehart said the Purcell guard has more than five years experience on the North Slope, had received the training and annual refresher courses and was current on all certifications.

"The guard as near as we know it intended to use a bean bag and thought he had and the bear took off," Rinehart said.

Still, the oil company has changed its policies regarding how polar bear hazing is conducted.

Rinehart said all ammunition will now be clearly marked by its type, with specific packaging colors and labels.

A "back-up bear hazer" also will be required to be on hand and verify that the correct ammunition for the level of hazing is about to be used, he said.

"We want to make completely sure that whatever guard is involved in a hazing incident knows exactly what type of hazing round is being used if it comes to that," Rinehart said.

The killing of the bear is highly unusual, especially given the number of bears that roam the oil field territory, he said.

The only other fatal incident was in 2002, also at Endicott, and security guards reported a bear that appeared to be starving, Rinehart said. Fish and Wildlife officers came out and euthanized the bear.

From 2005 through 2010, BP has recorded 541 polar bear observations, although that could include the same bear being observed more than once.

Bear hazing has occurred in 159 of those observations, he said.

Now, BP is working with the Fish and Wildlife Service on the investigation as well as continuing an in-house inquiry "to make sure we've got as full a picture as we can and to do everything we can to make sure this does not happen again," he said.